Technological advances of all kinds continue to present many complex ecological issues. Consequently, waste management and pollution prevention are two very significant challenges of the 21st century. The overwhelming quantity of plastic refuse has significantly contributed to the critical shortage of landfill space faced by many communities. For example, poly(ethylene terephthalate) (poly(oxy-1,2-ethanediyl-oxycarbonyl-1,4-diphenylenecarbonyl); “PET”), a widely used engineering thermoplastic for carpeting, clothing, tire cords, soda bottles and other containers, film, automotive applications, electronics, displays, etc. will contribute more than 1 billion pounds of waste to land-fills in 2002 alone. The worldwide production of PET has been growing at an annual rate of 10% per year, and with the increase in use in electronic and automotive applications, this rate is expected to increase significantly to 15% per year. Interestingly, the precursor monomers represent only about 2% of the petrochemical stream. Moreover, the proliferation of the use of organic solvents, halogenated solvents, water, and energy consumption in addressing the need to recycle commodity polymers such as PET and other polyesters has created the need for environmentally responsible and energy efficient recycling processes. See Nadkarni (1999) International Fiber Journal 14(3).
Significant effort has been invested in researching recycling strategies for PET, and these efforts have produced three commercial options; mechanical, chemical and energy recycling. Energy recycling simply burns the plastic for its calorific content. Mechanical recycling, the most widespread approach, involves grinding the polymer to powder, which is then mixed with “virgin” PET. See Mancini et al. (1999) Materials Research 2(1):33-38. Many chemical companies use this process in order to recycle PET at the rate of approximately 50,000 tons/year per plant. In Europe, all new packaging materials as of 2002 must contain 15% recycled material. However, it has been demonstrated that successive recycling steps cause significant polymer degradation, in turn resulting in a loss of desirable mechanical properties. Recycling using chemical degradation involves a process that depolymerizes a polymer to starting material, or at least to relative short oligomeric components. Clearly, this process is most desirable, but is the most difficult to control since elevated temperature and pressure are required along with a catalyst composed of a strong base, or an organometallic complex such as an organic titanate. See Sako et al. (1997) Proc. of the 4th Int'l Symposium on Supercritical Fluids, pp. 107-110. The use of such a catalyst results in significant quantities of undesirable byproducts, and materials processed by these methods are thus generally unsuitable for use in medical materials or food packaging, limiting their utility. Moreover, the energy required to effect depolymerization essentially eliminates sustainability arguments.
Accordingly, there is a need in the art for an improved depolymerization method. Ideally, such a method would not involve extreme reaction conditions, use of metallic catalysts, or a process that results in significant quantities of potentially problematic by-products.